For a vehicle equipped with an automatic transmission, the driver's convenience is enhanced if shifting of speeds is automatically executed according to driving conditions such as a running speed of the vehicle. Reduction of shift shock that may occur in the process of shifting is one criteria of quality of such an automatic transmission and performance of the vehicle.
In the case of an upshift of an automatic transmission, an engine speed at a target shift-speed (i.e., a target engine speed) is lower than a current engine speed. Therefore, a shift shock can be produced when an upshift is finished before the engine speed is lowered to close to the target engine speed at the target shift-speed.
In order to determine whether the engine speed is close to a target speed, a reference value is usually set. Therefore, when such a reference value may be preferably set, such a shift shock is expected to be reduced for various driving circumstances.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.